High-melting polyamides are understood according to the present invention to be polyamides having a melting point of at least 270° C. (DSC, ISO 11357). The inventive polyamides have an HDT A (1.8 MPa) of at least 260° C. when reinforced with at least 50 wt % glass fibers.
Reinforced polyamides play an increasing role in the field of technical construction materials because in addition to having a high stiffness they also have a good impact strength and high deflection temperature. Areas of application include, for example, interior and exterior parts in the automotive sector and in the area of other transport means, housing materials for appliances and equipment for telecommunications, entertainment electronics, household appliances, mechanical engineering equipment, equipment in the heating field and fastening parts for installations. In parts for the automotive sector, for example, metal-like properties which are achieved only through highly filled reinforced molding compounds, however, are important. In the case of parts with thin walls, a high flow length of the molding compounds in particular is necessary, but this cannot be achieved at all or can only be achieved poorly when using conventional long and short glass fibers.
The particular advantage of reinforced polyamides is also the extraordinarily good bond between the polymer matrix and the reinforcing materials. This is also true even at high degrees of reinforcement leading to products with a high tensile E modulus. The impact strength, the transverse strength and stiffness as well as the deformation and processability of the products of the state of the art do not, however, meet all requirements, in particular when at the same time a high thermal stability and high deflection temperature are required.
In the present patent application, polyamides as used below are understood to refer to polymers whose basic building blocks are held together by amide linkages (NH—CO) and which can be synthesized by polycondensation or polymerization of monomers, e.g., dicarboxylic acids, dicarboxylic acid halides, dinitriles, diamines, aminocarboxylic acids and/or lactams. These may be homo- or copolyamides or mixtures thereof. The number average molecular weight of the polyamides should be more than 5,000, preferably more than 8,000 but less than 30,000, in particular less than 20,000 g/mol, corresponding to solution viscosities of ηrel of less than 2.3, in particular ηrel of less than 1.9, especially preferably ηrel of less than 1.8 (measured in m-cresol, 0.5 wt %, 20° C.).
EP 0 196 194 B1 describes a strand comprising a plurality of individual filaments of glass having a noncircular cross section as well as the production thereof. The cross section of the glass fibers may be oval, elliptical, elliptical with constriction(s) (so-called cocoon fibers) or polygonal.
EP 0 246 620 B1 describes an object comprised of a glass fiber-reinforced thermoplastic material such that the glass fibers have a rectangular, elliptical or cocoon-shaped cross section. The cocoon-shaped cross section is understood in EP 0 246 620 B1 to refer to glass fibers having an elongated or oval shape or a curved shape, each with a constricted section. These glass fibers have an aspect ratio of 2.
EP 0 376 616 B1 describes a thermoplastic polymer comparison comprising a thermoplastic material and 1 to 65% of a fibrous reinforcement with a noncircular cross section, such that the cross-sectional area and the ratio of the cross-sectional axes of the reinforcing fibers perpendicular to one another are characterized in greater detail. The cross section of the reinforcing fibers has a curved or half-round contour. The composition is characterized by a high dimensional stability and low deformation.
To reduce the deformation of molded articles produced under thermoplastic conditions, according to JP 10219026 A2, the thermoplastic matrix is reinforced with a mixture of glass fibers having a circular cross section and glass fibers having a flat cross section. In the single example given in this document, polyamide 66 (nylon 66) is mentioned as the polymer matrix.
JP 2006045390 A2 describes granules reinforced with long glass fibers, comprising a thermoplastic matrix and up to 60 wt % glass fibers with a flat cross section. The granule length and the fiber length are identical. Advantageous properties of the molded articles produced from the reinforced composition according to JP 2006045390 A2 include a good surface quality and a high impact strength.
JP 8259808 A2 describes polyamide molding compounds comprising 50 to 80 wt % of a polyamide resin, preferably a polyamide 6 having a relative viscosity of 2.5 to 4.0 and 20 to 50 wt % flat fibers plus optionally up to 30 wt % of a filler.
EP 1 788 026 B1 describes reinforced polyamide molding compounds comprised of a blend of polyamide 66 and a copolyamide 6T/6I. A mixture of glass fibers and carbon fibers is used as the reinforcing material.